Fuel injectors for gas turbine engines on an aircraft direct fuel from a manifold to a combustion chamber. The fuel injector typically has an inlet fitting connected to the manifold for receiving the fuel, a fuel spray nozzle located within the combustion chamber of the engine for atomizing (dispensing) the fuel, and a housing stem extending between and fluidly interconnecting the inlet fitting and the fuel nozzle. Appropriate check valves and/or flow dividers can be disposed within the fuel nozzle to control the flow of fuel through the nozzle. The fuel injector has an attachment flange which enables multiple injectors to be attached to the combustor casing of the engine in a spaced-apart manner around the combustor to dispense fuel in a generally cylindrical pattern.
Fuel injectors are typically heatshielded because of the high operating temperatures within the engine casing. High temperature gas turbine compressor discharge air flows around the housing stem of the fuel injector before entering the combustor. The heat shielding prevents the fuel passing through the injector from breaking down into its constituent components (i.e., "coking"), which occurs when the wetted wall temperatures of a fuel passage exceed 400.degree. F. The coke in the fuel passages of the fuel injector can build up to restrict fuel flow to the nozzle.
One type of heatshield assembly for a fuel injector has an internal heatshield disposed within the fuel passage of the housing stem. The internal heatshield comprises a straight fuel conduit which is rigidly attached at one end to either the fuel nozzle or the inlet fitting, and is left unattached at the other end to allow for differences in thermal expansion between the relatively cooler inner heatshield and the hotter outer housing stem. The unattached end has a small clearance within the bore of the stem which allows for fuel to enter the cavity between the heatshield and the internal walls of the housing stem. Over time, the fuel in this cavity cokes to provide an insulating layer between the housing stem and the fuel conduit. While this technique for heatshielding is appropriate for some applications, the insulating coke layer can take a number of engine cycles to form, and the resulting coke layer can migrate into the fuel stream, which can affect downstream fuel passages.
Another type of heatshield assembly for a fuel injector has an external heatshield around the housing stem. This heatshield typically includes a pair of outer U-shaped heatshield members which are located on opposite sides of the housing stem, and extend axially herealong. The heatshield members are secured together along their opposite abutting side edges, and to the housing stem, such as by welding or brazing. The heatshield members define a stagnant air gap between the heatshield members and the outer surface of the housing stem. It is believed that the stagnant air gap between the heatshield members provides better insulating characteristics than a coke or carbon-filled gap. While this type of heatshield assembly can also be appropriate in certain applications, the use of external heatshield members increases the number of components for the fuel injector, which thereby increases material costs, assembly time, and hence the overall cost of the fuel injector. There can also be issues with the attachment of the heatshield members to the housing stem because of the thermal expansion characteristics of the outer heatshield members. This can limit the useful life of the fuel injectors over constant engine cycling.
It is known to provide an internal heatshield comprising a straight fuel conduit with both ends of the conduit sealed to the housing stem. In this case, a stagnant air gap is created between the conduit and the internal walls of the housing stem. To compensate for the thermal expansion characteristics of the heatshield and the housing stem, it is known that at least one end of the conduit can include a metal bellows or a slip-fit attachment with one or more O-ring seals to allow for thermal expansion of the conduit with respect to the housing stem. The other end of the conduit is typically rigidly attached to the housing stem. It is believed that both ends have not been rigidly attached to the housing stem in the past because of concerns of early fatigue failures over repeated engine cycling due to the thermal expansion characteristics of the conduit. While the stagnant air gap provides better insulating characteristics than a coke or carbon-filled gap, it is believed that a leak path can develop over time around the O-rings, particularly at elevated temperatures. Using O-rings and metal bellows can also increase the number of components associated with the fuel injector, and can be complicated and time-consuming to assemble, thereby also increasing the over-all cost of the fuel injector.
Thus it is believed there is a demand in the industry for a further improved fuel injector for gas turbine engines which maintains fuel passage wetted wall temperatures within the housing stem below the coking threshold, which has few components which are relatively straight-forward to manufacture and assemble, and which maintains reliable, leak-free operation over multiple cycles of the aircraft engine.